What is the ruling on Salat al-Istikhara if it leads to reassurance and a good dream concerning an matter that is impermissible in Sharia, such as a girl conversing with a young man? And what is the correct action to take in this situation, especially given the girl's young age and her need to complete her studies, and her parents' disapproval of marriage before that?

Salat al-Istikhara (prayer for guidance) is performed for permissible matters and is not prescribed for what is forbidden, obligatory, recommended, or disliked.

The questioner must stop speaking with the non-mahram young man, disregard him, and cease thinking about him, because relationships and love outside the bounds of marriage are forbidden.

What you see in dreams is not to be given weight; it is merely confused dreams and whispers from Satan.

Studying does not prevent marriage if the questioner finds a suitable partner. She must inform her family of her desire to marry to protect her chastity. If her guardian prevents her from marrying a suitable partner, she may refer her case to a judge. However, it is better to be patient until she completes her studies and satisfies her parents.